OPERATING SYSTEMS
PROCESS CONTROL BLOCK
Software Engr. Imdad Manik
03007060570

Role of the Process Control Block
– Process Control block is used for :
– storing the collection of information about the processes.
– it is also called Data Structure
– various information of PCB as following:
i. Name of the process
ii. State of the process mean Ready, Activate, Wait
iii. Resources allocated to the process
iv. Memory which is provided to the process
v. Scheduling information
vi. I /O devices used by the process
vii.Process ID , given by the CPU when a process request for a
service.

Requirements of an
Operating System
• Fundamental Task: Process Management
• The Operating System must
– Interleave the execution of multiple processes
– Allocate resources to processes, and protect
the resources of each process from other
processes,
– Enable processes to share and exchange
information,
– Enable synchronization among processes.

Concepts
• From earlier chapters we saw:
– Computer platforms consists of a collection of
hardware resources
– Computer applications are developed to
perform some task
– It is inefficient for applications to be written
directly for a given hardware platform

Concepts cont…
– OS provides an interface for applications to
use
– OS provides a representation of resources
that can be requested and accessed by
application

The OS Manages
Execution of Applications
• Resources are made available to multiple
applications
• The processor is switched among multiple
application
• The processor and I/O devices can be
used efficiently

What is a “process”?
• A program in execution
• An instance of a program running on a
computer
• The entity that can be assigned to and
executed on a processor
• A unit of activity characterized by the
execution of a sequence of instructions, a
current state, and an associated set of
system instructions

Process Elements
• A process is comprised of:
– Program code (possibly shared)
– A set of data
– A number of attributes describing the state of
the process

Process Elements
• While the process is running it has a
number of elements including
– Identifier
– State
– Priority
– Program counter
– Memory pointers
– Context data
– I/O status information
– Accounting information

Process Control Block
• Contains the process
elements
• Created and managed
by the operating system
• Allows support for
multiple processes

Trace of the Process
• The behavior of an individual process is
shown by listing the sequence of
instructions that are executed
• This list is called a Trace
• Dispatcher is a small program which
switches the processor from one process
to another

Process Execution
• Consider three
processes being
executed
• All are in memory
(plus the dispatcher)
• Lets ignore virtual
memory for this.

Trace from the
processes point of view:
• Each process runs to completion

Trace from Processors
point of view
Timeout
I/O

Roadmap
– How are processes represented and
controlled by the OS.
– Process states which characterize the
behaviour of processes.
– Data structures used to manage processes.
– Ways in which the OS uses these data
structures to control process execution.
– Discuss process management in UNIX SVR4.

Two-State Process Model
• Process may be in one of two states
– Running
– Not-running

Queuing Diagram
Etc … processes moved by the dispatcher of the OS to the CPU then back
to the queue until the task is competed

Process Birth and Death
Creation
New batch job
Interactive Login
Created by OS to
provide a service
Termination
Normal Completion
Memory unavailable
Protection error
Spawned by existing Operator or OS
process
Intervention
See tables 3.1 and 3.2 for more

Process Creation
• The OS builds a data structure to manage
the process
• Traditionally, the OS created all processes
– But it can be useful to let a running process
create another
• This action is called process spawning
– Parent Process is the original, creating,
process
– Child Process is the new process

Process Termination
• There must be some way that a process
can indicate completion.
• This indication may be:
– A HALT instruction generating an interrupt
alert to the OS.
– A user action (e.g. log off, quitting an
application)
– A fault or error
– Parent process terminating

Five-State
Process Model

Using Two Queues

Multiple Blocked Queues

Suspended Processes
• Processor is faster than I/O so all
processes could be waiting for I/O
– Swap these processes to disk to free up more
memory and use processor on more
processes
• Blocked state becomes suspend state
when swapped to disk
• Two new states
– Blocked/Suspend
– Ready/Suspend

One Suspend State

Two Suspend States

Reason for Process
Suspension
Reason
Comment
Swapping
The OS needs to release sufficient main memory to
bring in a process that is ready to execute.
Other OS Reason
OS suspects process of causing a problem.
Interactive User
Request
e.g. debugging or in connection with the use of a
resource.
Timing
A process may be executed periodically (e.g., an
accounting or system monitoring process) and may
be suspended while waiting for the next time.
Parent Process
Request
A parent process may wish to suspend execution of
a descendent to examine or modify the suspended
process, or to coordinate the activity of various
descendants.
Table 3.3 Reasons for Process Suspension

Roadmap
– How are processes represented and
controlled by the OS.
– Process states which characterize the
behaviour of processes.
– Data structures used to manage processes.
– Ways in which the OS uses these data
structures to control process execution.
– Discuss process management in UNIX SVR4.

Processes
and Resources

Operating System
Control Structures
• For the OS is to manage processes and
resources, it must have information about
the current status of each process and
resource.
• Tables are constructed for each entity the
operating system manages

OS Control Tables

Memory Tables
• Memory tables are used to keep track of
both main and secondary memory.
• Must include this information:
– Allocation of main memory to processes
– Allocation of secondary memory to processes
– Protection attributes for access to shared
memory regions
– Information needed to manage virtual memory

I/O Tables
• Used by the OS to manage the I/O
devices and channels of the computer.
• The OS needs to know
– Whether the I/O device is available or
assigned
– The status of I/O operation
– The location in main memory being used as
the source or destination of the I/O transfer

File Tables
• These tables provide information about:
– Existence of files
– Location on secondary memory
– Current Status
– other attributes.
• Sometimes this information is maintained
by a file management system

Process Tables
• To manage processes the OS needs to
know details of the processes
– Current state
– Process ID
– Location in memory
– etc
• Process control block
– Process image is the collection of program.
Data, stack, and attributes

Process Attributes
• We can group the process control block
information into three general categories:
– Process identification
– Processor state information
– Process control information

Process Identification
• Each process is assigned a unique
numeric identifier.
• Many of the other tables controlled by the
OS may use process identifiers to crossreference process tables

Processor State
Information
• This consists of the contents of processor
registers.
– User-visible registers
– Control and status registers
– Stack pointers
• Program status word (PSW)
– contains status information
– Example: the EFLAGS register on Pentium
processors

Pentium II
EFLAGS Register
Also see Table 3.6

Process Control
Information
• This is the additional information needed
by the OS to control and coordinate the
various active processes.
– See table 3.5 for scope of information

Structure of Process
Images in Virtual Memory

Role of the
Process Control Block
• The most important data structure in an
OS
– It defines the state of the OS
• Process Control Block requires protection
– A faulty routine could cause damage to the
block destroying the OS’s ability to manage
the process
– Any design change to the block could affect
many modules of the OS

Roadmap
– How are processes represented and
controlled by the OS.
– Process states which characterize the
behaviour of processes.
– Data structures used to manage processes.
– Ways in which the OS uses these data
structures to control process execution.
– Discuss process management in UNIX SVR4.

Modes of Execution
• Most processors support at least two
modes of execution
• User mode
– Less-privileged mode
– User programs typically execute in this mode
• System mode
– More-privileged mode
– Kernel of the operating system

Process Creation
• Once the OS decides to create a new
process it:
– Assigns a unique process identifier
– Allocates space for the process
– Initializes process control block
– Sets up appropriate linkages
– Creates or expand other data structures

Switching Processes
• Several design issues are raised
regarding process switching
– What events trigger a process switch?
– We must distinguish between mode switching
and process switching.
– What must the OS do to the various data
structures under its control to achieve a
process switch?

When to switch processes
A process switch may occur any time that the OS has gained control from the
currently running process. Possible events giving OS control are:
Mechanism
Cause
Use
Interrupt
External to the execution of
the current instruction
Reaction to an asynchronous
external event
Trap
Associated with the execution
of the current instruction
Handling of an error or an
exception condition
Supervisor call
Explicit request
Call to an operating system
function
Table 3.8 Mechanisms for Interrupting the Execution of a Process

Change of
Process State …
• The steps in a process switch are:
1. Save context of processor including program
counter and other registers
2. Update the process control block of the
process that is currently in the Running state
3. Move process control block to appropriate
queue – ready; blocked; ready/suspend

Change of
Process State cont…
4. Select another process for execution
5. Update the process control block of the
process selected
6. Update memory-management data
structures
7. Restore context of the selected process

Processes in Unix

UNIX Process State
Transition Diagram

UNIX Process States

A Unix Process
• A process in UNIX is a set of data
structures that provide the OS with all of
the information necessary to manage and
dispatch processes.
• See Table 3.10 which organizes the
elements into three parts:
– user-level context,
– register context, and
– system-level context.

Process Creation
• Process creation is by means of the kernel
system call,fork( ).
• This causes the OS, in Kernel Mode, to:
1. Allocate a slot in the process table for the
new process.
2. Assign a unique process ID to the child
process.
3. Copy of process image of the parent, with
the exception of any shared memory.

Process Creation
cont…
4. Increment the counters for any files owned
by the parent, to reflect that an additional
process now also owns those files.
5. Assign the child process to the Ready to
Run state.
6. Returns the ID number of the child to the
parent process, and a 0 value to the child
process.

After Creation
• After creating the process the Kernel can
do one of the following, as part of the
dispatcher routine:
– Stay in the parent process.
– Transfer control to the child process
– Transfer control to another process.